You’re alone in a rail yard when suddenly you spot a runaway trolley. Looking around for help, you notice five workers all tied up on the track in the careening car’s path. There is a lever nearby. Pull it, and the trolley will divert to a sidetrack saving the five. But there is a man on the sidetrack who will be hit and killed. What do you do?

Joshua Greene revolutionized the study of morality by scanning the brains of individuals while they puzzled over philosophical questions such as this one. From this research, Greene made stunning discoveries about how individuals contemplate issues of right and wrong and how different neural networks respond differently depending on the degree of personal involvement in a dilemma.

What is therefore surprising and remarkable about “Moral Tribes,’’ Greene’s first book, is his evolution from a focus on the individual and personal to the collective and communal. Drawing on his background in philosophy, psychology, and neuroscience, Greene, who directs Harvard’s moral cognition lab, uses insights from these initial studies to argue that the primary reason morality matters is not to provide a guide for an individual. Rather, as he explains, morality matters as a road map for how individuals can live successful social lives within communities and how communities can interact effectively with one another.

Advertisement

Greene first presents two problems that his theory of morality must answer. Then he dives into the thought experiments and scientific evidence that have guided him to a solution. And lest you think Greene’s is a purely academic exercise at the end of this book he goes on to apply his ideas to some of the nation’s most vexing political issues.

Get The Weekender in your inbox:

The Globe's top picks for what to see and do each weekend, in Boston and beyond.

Greene suggests that the two central problems of modern existence are both inherently social, and he explains the brain’s ability to respond to each. The first problem involves the well-known tragedy of the commons, in which multiple individuals (herders) share, and ultimately deplete, a common resource (pasture), each by acting in his or her own self-interest without regard to the group.

Greene argues this problem is far from a tragedy because humans’ intuitive mode of thinking — what Greene refers to as our “point-and-shoot settings” — produces emotional responses such as guilt, gratitude, and empathy that nudge humans toward cooperation. That is, our gut-level decisions tend to favor the group over the individual.

The second problem is far more difficult to overcome and ironically stems from the way we resolve the tragedy of the commons. This dilemma is what Greene terms the tragedy of common-sense morality in which different communities clash on the fairest way to coordinate.

This is the big one, the mess into which we are thrust precisely because our point-and-shoot settings are so good at binding us into groups. The downside is that the goodwill and loyalty expressed toward those within our circles come at the expense of caring for those outside them.

Advertisement

Characterizing intergroup conflict this way explains why the Democratic-Republican rivalry is as intractable as Yankees-Red Sox. What it means to favor one side’s shared values is inherently to despise the other’s.

A big problem requires a big solution, one that relies far more on deliberation and reason than intuition and emotion and one that shifts our automatic point-and-shoot settings into “manual mode.” For Greene, this big, hairy, cognitively demanding solution is deep pragmatism, a modern rebranding of Jeremy Bentham and J.S. Mill’s utilitarianism that commands us to “maximize happiness impartially.”

Greene understands the historical unpopularity of this idea and comes ready to do battle. In fact, he argues in favor of deep pragmatism in such a fluid “isn’t this patently obvious” way, that it is not until the book’s penultimate chapter that I realized that his solution is a politically radical idea.

In this chapter, Greene wades into the muck of American politics to use deep pragmatism to provide a mathematically precise defense of the pro-choice position as a test case. Even more ambitiously, Greene defends American liberalism as an ideology that employs deep pragmatism to overcome our tribal instincts.

As much as Greene credits emotions for their role in converting self-interested concerns into collective ones, ultimately he is a champion of reason. It is reason, Greene argues, that enables us to perform the cost-benefit calculations that determine how to maximize happiness.

Advertisement

Thus, “Moral Tribes’’ offers a Bentham-Mill mash-up for the 21st century, with the benefit of a number of tools such as functional neuroimaging, the implicit association test, and oxytocin nasal spray to which Bentham and Mill never had access.

Perhaps Greene’s most radical idea of all is that there is a “best” morality. Many psychologists are hesitant to offer prescriptive advice, especially when it comes to matters of morality, preferring to let the data speak for themselves.

On the other hand, philosophers often face criticism for their willingness to dole out prescriptions without relying on a shred of actual data. For these reasons, Greene’s interdisciplinary background serves him well, allowing him to make his case for reason, with reason.

Toggling between big ideas, technical details, and his personal intellectual journey, Greene writes a thesis suitable to both airplane reading and PhD seminars. Greene also takes his time to argue his case rather than flashing quick tips, easy answers, or byte-size solutions. “Moral Tribes’’ offers a psychology far beyond the realm of self-help, instead probing the intricacy and complexity of morality in an attempt to help, and perhaps unite, entire communities.

Adam Waytz is a psychologist and professor of management and organizations at the Kellogg School of Management at Northwestern University. He teaches a class about ethics to MBA students.